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EXAMPLE APPLICATION OF CFD SIMULATIONS FOR SHORT-RANGE ATMOSPHERIC DISPERSION OVER THE OPEN FIELDS OF PROJECT PRAIRIE GRASS
Citation:
TANG, W., A. H. HUBER, B. BELL, K. KUEHLERT, AND W. SCHWARZ. EXAMPLE APPLICATION OF CFD SIMULATIONS FOR SHORT-RANGE ATMOSPHERIC DISPERSION OVER THE OPEN FIELDS OF PROJECT PRAIRIE GRASS. Presented at Air and Waste Management Association 98th Annual Conference, Minneapolis, MN, June 21 - 25, 2005.
Impact/Purpose:
The objective of this task is to improve EPA's ability to accurately predict the concentrations and deposition of air pollutants in the atmosphere that are known or suspected to cause cancer or other serious health effects to humans, or adverse environmental effects. It is an essential component of EPA's National Air Toxics Assessment (NATA), which seeks to identify and quantify the concentrations and sources of those hazardous air pollutants which are of greatest potential concern, in terms of contribution to population risk. It is a major contributor to NERL's Air Toxics Research Program.
"Air toxics" or "hazardous air pollutants" (HAPs) is a category that covers a large variety of chemicals, which range from relatively non reactive to extremely reactive; can exist in the gas, aqueous, and/or particle phases; display a large range of volatilities; experience varying deposition velocities, including in some cases revolatilization; and are emitted from a wide variety of sources at a large variety of different scales. In addition, concentrations of air toxics are needed by regulators for both short (days) as well as long (up to a year) time scales. These requirements challenge our current capabilities in air quality models far beyond the needs for other pollutants, such as ozone. The specific work being done under this task involves 1.) developing and testing chemical mechanisms which are appropriate for describing the chemistry of air toxics; 2.) incorporating these chemical and physical mechanisms into EPA's CMAQ modeling system and applying the model at a variety of scales; and 3.) developing the methods for using models to predict HAPs concentrations at subgrid or neighborhood scales; and 4.) using these tools to assess the magnitude and variability of concentrations to which urban populations are exposed.
Description:
Computational Fluid Dynamics (CFD) techniques are increasingly being applied to air quality modeling of short-range dispersion, especially the flow and dispersion around buildings and other geometrically complex structures. The proper application and accuracy of such CFD techniques needs to be assessed. The first step in such an assessment is to examine the ability to simulate atmospheric-like boundary layer flow in absence of buildings and plume dispersion over open fields.
Case studies based on the Project Prairie Grass field program were used to develop and evaluate CFD simulation of plume dispersion over an open field. A commercial CFD code, FLUENT has been used in this study. For a given ground roughness height and friction velocity, a 2D CFD simulation was first developed to generate an atmosphere-like boundary layer. The simulated mean velocity profile matched field measurements. This atmosphere-like boundary layer was then applied as a boundary condition in the 3D CFD simulation of each dispersion case study. The simulated profiles of mean velocity, turbulent kinetic energy and turbulent dissipation rate were maintained within the downwind domain for the CFD simulation.
The steady-state CFD simulation was used to model the time-averaged plume by applying wind direction temporal data to account for variations in wind direction. Results for field runs characterized by near thermally neutral stability conditions were evaluated thoroughly in comparison with the field-measured plume from point source release of a tracer gas. The CFD model performed well as demonstrated by the reported performance metrics, based on comparisons with not only the measured centerline concentrations but also the full lateral and vertical profiles.
This paper demonstrates that CFD methods can well simulate the atmosphere-like boundary layer and plume dispersion over an open plain. The authors believe this is a critical first step before application to more complex situations with flow and dispersion around buildings and other geometrically complex structures. The goal of our ongoing research is to demonstrate application to these complex situations where the Gaussian plume models definitely fail. Work is ongoing to extend application of the methods presented in this paper to all stability categories and to flow involving building influences.